Various fluorochemical wet pick-up and internal sizing agents for paper treatment are described, for example, in Rengel and Young, "Internal Sizing of Paper and Paperboard", Tappi monograph series number 33, pp. 170-189 (1971), Colbert, "Fluorochemicals-Fluid Repellency for Non-woven Substrates", Tappi, The Journal of the Technical Association of the Pulp and Paper Industry, 59, 9, (September, 1976), Banks, Ed., Organofluorine Chemicals and their Industrial Applications, pp. 231-234 (1979), and Schwartz, "Oil Resistance Utilizing Fluorochemicals", Tappi conference preprint, 1980 Sizing Short Course, Atlanta, Ga. Several fluorochemical phosphates have been approved by the U.S. Food and Drug Administration for use on paperboard in direct contact with food for human consumption. These fluorochemical phosphates can be used as wet pick-up or as internal treatments. They primarily provide oil resistance, and are used on paper plates, bags for bakery goods, cartons and trays for oil fried foods (e.g., French fries), and in bags and cartons for pet foods.
The advent in recent years of the microwave oven has created a need for non-metallic containers for cooking or food-warming which have resistance to both oily and aqueous foods at oven temperatures, since metallic containers (e.g., aluminum trays) do not efficiently cook foods in microwave ovens and may promote electrical arcing if the metallic container walls approach or touch the walls of the microwave oven. A suitable non-metallic food container should also withstand freezing temperatures and conventional oven temperatures because foods solid in such containers will often be frozen and will be cooked in both microwave and conventional ovens. Cooking times for foods stored in such containers usually range from a few minutes to sixty minutes or more, and cooking temperatures usually range from about 95.degree. C. to 240.degree. C. or higher.
Existing commercially available fluorochemical phosphate sizing treatments do not provide sufficient high temperature water repellency to ovenable paperboard food trays exposed to cooking conditions for extended periods of time. Food packagers have had to employ other container materials or constructions to obtain adequate ovenability. For example, former food trays can be made entirely from plastics such as polyethylene terephthalate. Also laminated, stamped food trays can be made from a layer of conventional paperboard coated on the food side with a thin (0.25 to 0.33 millimeters) film of extruded polyester. Food containers made entirely from plastic are relatively expensive, consume scarce petroleum resources, and lose rigidly at elevated temperatures. Food containers made from laminated paperboard and polyester sheets can become stained with oil on the unprotected outside surface (e.g., during food filling operations), are prone to rupture of the container at corners during tray-forming operations (due in part to differences in moisture content within the paperboard sheet), are susceptible to delamination when foods are heated to very high temperatures (e.g., when bacon or sausage are heated in microwave or conventional ovens), and are relatively expensive, requiring about 110 grams of polyester resin per kilogram of paperboard. In addition, scrap or waste paperboard from tray forming or other operations performed on paperboard/polyester laminate is not repulpable, and this scrap is sometimes as much as 25 percent of the total paperboard/polyester laminate consumed.